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The parent compounds of the copper oxide high-transition-temperature (high- T c ) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are ‘doped’ away from stoichiometry 1 . For the compound Bi 2 Sr 2 CaCu 2 O 8+ x , doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers (‘holes’) into the CuO 2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high- T c superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi 2 Sr 2 CaCu 2 O 8+ x , on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of ∼14 Å. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.

An experimental realization of a photonic topological insulator is reported that consists of helical waveguides arranged in a honeycomb lattice; the helicity provides a symmetry-breaking effect, leading to optical states that are topologically protected against scattering by disorder. A photonic topological insulator One of the hottest fields of condensed-matter research is that of topological insulators. They exist in electronic states that are robust against disorder owing to the topological protection provided by the underlying electronic structure. Their potential practical importance lies in their ability to control and manipulate electron waves without scattering. An interesting question is whether it would be possible to make a topological insulator for light. The answer is yes, and here Mordechai Segev and colleagues demonstrate the first experimental realization of a photonic topological insulator, which consists of helical waveguides arranged in a honeycomb lattice. The helicity is crucial, providing a symmetry breaking effect leading to topological insulator properties. The authors demonstrate one-way edge states that are protected from scattering. Topological insulators are a new phase of matter 1 , with the striking property that conduction of electrons occurs only on their surfaces 1 , 2 , 3 . In two dimensions, electrons on the surface of a topological insulator are not scattered despite defects and disorder, providing robustness akin to that of superconductors. Topological insulators are predicted to have wide-ranging applications in fault-tolerant quantum computing and spintronics. Substantial effort has been directed towards realizing topological insulators for electromagnetic waves 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 . One-dimensional systems with topological edge states have been demonstrated, but these states are zero-dimensional and therefore exhibit no transport properties 11 , 12 , 14 . Topological protection of microwaves has been observed using a mechanism similar to the quantum Hall effect 15 , by placing a gyromagnetic photonic crystal in an external magnetic field 5 . But because magnetic effects are very weak at optical frequencies, realizing photonic topological insulators with scatter-free edge states requires a fundamentally different mechanism—one that is free of magnetic fields. A number of proposals for photonic topological transport have been put forward recently 6 , 7 , 8 , 9 , 10 . One suggested temporal modulation of a photonic crystal, thus breaking time-reversal symmetry and inducing one-way edge states 10 . This is in the spirit of the proposed Floquet topological insulators 16 , 17 , 18 , 19 , in which temporal variations in solid-state systems induce topological edge states. Here we propose and experimentally demonstrate a photonic topological insulator free of external fields and with scatter-free edge transport—a photonic lattice exhibiting topologically protected transport of visible light on the lattice edges. Our system is composed of an array of evanescently coupled helical waveguides 20 arranged in a graphene-like honeycomb lattice 21 , 22 , 23 , 24 , 25 , 26 . Paraxial diffraction of light is described by a Schrödinger equation where the propagation coordinate ( z ) acts as ‘time’ 27 . Thus the helicity of the waveguides breaks z -reversal symmetry as proposed for Floquet topological insulators. This structure results in one-way edge states that are topologically protected from scattering.

Two general features of a superconductor, which appear at the critical temperature, are the formation of an energy gap and the expulsion of magnetic flux (the Meissner effect). In underdoped copper oxides, there is strong evidence that an energy gap (the pseudogap 1 ) opens up at a temperature significantly higher than the critical temperature (by 100–220 K). Certain features of the pseudogap suggest that it is closely related to the gap that appears at the critical temperature (for example, the variation of the gap magnitudes around the Fermi surface and their maximum amplitudes are very similar 2 , 3 ). However, the Meissner effect is absent in the pseudogap state. The nature of the pseudogap state, and its relation (if any) to the superconducting state are central issues in understanding copper oxide superconductivity. Recent evidence suggests that, in the underdoped regime, the Meissner state is destroyed above the critical temperature by strong phase fluctuations 1 , 4 , 5 , 6 , 7 (as opposed to a vanishing of the superfluid density). Here we report evidence for vortices (or vortex-like excitations) in La 2- x Sr x CuO 4 at temperatures significantly above the critical temperature. A thermal gradient is applied to the sample in a magnetic field. Vortices are detected by the large transverse electric field produced as they diffuse down the gradient (the Nernst effect). We find that the Nernst signal is anomalously enhanced at temperatures as high as 150 K.

Although the crystal structures of the copper oxide high-temperature superconductors are complex and diverse, they all contain some crystal planes consisting of only copper and oxygen atoms in a square lattice: superconductivity is believed to originate from strongly interacting electrons in these CuO 2 planes. Substituting a single impurity atom for a copper atom strongly perturbs the surrounding electronic environment and can therefore be used to probe high-temperature superconductivity at the atomic scale. This has provided the motivation for several experimental 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 and theoretical studies 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 . Scanning tunnelling microscopy (STM) is an ideal technique for the study of such effects at the atomic scale, as it has been used very successfully to probe individual impurity atoms in several other systems 21 , 22 , 23 , 24 , 25 . Here we use STM to investigate the effects of individual zinc impurity atoms in the high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+δ . We find intense quasiparticle scattering resonances 26 at the Zn sites, coincident with strong suppression of superconductivity within ∼15 Å of the scattering sites. Imaging of the spatial dependence of the quasiparticle density of states in the vicinity of the impurity atoms reveals the long-sought four-fold symmetric quasiparticle ‘cloud’ aligned with the nodes of the d -wave superconducting gap which is believed to characterize superconductivity in these materials.

Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states, in which electrons have their spin locked at a right angle to their momentum under the protection of time-reversal symmetry. The topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking, such as that observed in superconductivity, can lead to new quantum phenomena and devices. We fabricated a superconducting TI/superconductor heterostructure by growing dibismuth triselenide (Bi(2)Se(3)) thin films on superconductor niobium diselenide substrate. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we observed the superconducting gap at the Bi(2)Se(3) surface in the regime of Bi(2)Se(3) film thickness where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana fermions in condensed matter physics.

Optical data are reported on a spectral weight transfer over a broad frequency range of Bi2Sr2CaCu2O8+delta, when this material became superconducting. Using spectroscopic ellipsometry, we observed the removal of a small amount of spectral weight in a broad frequency band from 10(4) cm(-1) to at least 2 x 10(4) cm(-1), due to the onset of superconductivity. We observed a blue shift of the ab-plane plasma frequency when the material became superconducting, indicating that the spectral weight was transferred to the infrared range. Our observations are in agreement with models in which superconductivity is accompanied by an increased charge carrier spectral weight. The measured spectral weight transfer is large enough to account for the condensation energy in these compounds.

Iron oxypnictides: The hottest thing in superconductivity The hunt for new materials exhibiting high-temperature superconductivity is on again. A complex iron-based oxide, containing lanthanum and arsenic, was recently found to exhibit a transition temperature ( T c ) of about 26 K when doped with fluoride ions. That's respectable, but far from the heights achieved in copper oxide superconductors. Now Takahashi et al . show that the application of around 40,000 atmospheres of pressure can raise the T c of this material substantially, to about 43 K. This is the highest t c yet reported for a non-copper-based material. What is more, this record is unlikely to last for long: the complexity of 'iron oxypnictides' of this type offers considerable flexibility for chemical modification, and we can expect to hear of yet higher transition temperatures. This paper — and the prospect of a new wave of superconductor fever — is the subject of an Editorial in the 24 April issue of Nature ( 452 , 914; 2008). The application of pressure can raise the superconducting transition temperature of oxypnictide (a pnicogen being a group V element) substantially, to a maximum value of about 43 K. This is the highest transition temperature yet reported for a non-copper-based material, but this record is unlikely to last for long: the material system offers considerable flexibility for chemical modification, and we can reasonably anticipate that this record will soon be superseded. The iron- and nickel-based layered compounds LaOFeP (refs 1 , 2 ) and LaONiP (ref. 3 ) have recently been reported to exhibit low-temperature superconducting phases with transition temperatures T c of 3 and 5 K, respectively. Furthermore, a large increase in the midpoint T c of up to ∼26 K has been realized 4 in the isocrystalline compound LaOFeAs on doping of fluoride ions at the O 2- sites (LaO 1- x F x FeAs). Experimental observations 5 , 6 and theoretical studies 7 , 8 , 9 suggest that these transitions are related to a magnetic instability, as is the case for most superconductors based on transition metals. In the copper-based high-temperature superconductors, as well as in LaOFeAs, an increase in T c is often observed as a result of carrier doping in the two-dimensional electronic structure through ion substitution in the surrounding insulating layers, suggesting that the application of external pressure should further increase T c by enhancing charge transfer between the insulating and conducting layers. The effects of pressure on these iron oxypnictide superconductors may be more prominent than those in the copper-based systems, because the As ion has a greater electronic polarizability, owing to the covalency of the Fe–As chemical bond, and, thus, is more compressible than the divalent O 2- ion. Here we report that increasing the pressure causes a steep increase in the onset T c of F-doped LaOFeAs, to a maximum of ∼43 K at ∼4 GPa. With the exception of the copper-based high- T c superconductors, this is the highest T c reported to date. The present result, together with the great freedom available in selecting the constituents of isocrystalline materials with the general formula LnOTMPn (Ln, Y or rare-earth metal; TM, transition metal; Pn, group-V, ‘pnicogen’, element), indicates that the layered iron oxypnictides are promising as a new material platform for further exploration of high-temperature superconductivity.

The photoemission line shpapes of the optimally doped cuprate Bi2Sr2CaCu2O8+Delta were studied in the direction of a node in the superconducting order parameter by means of very high resolution photoemission spectroscopy.

The discovery 1 of high-temperature superconductivity in copper oxides raised the possibility that superconductivity could be achieved at room temperature. But since 1993, when a critical temperature ( T c ) of 133 K was observed in the HgBa 2 Ca 2 Cu 3 O 8+δ ( ref. 2 ), no further progress has been made in raising the critical temperature through material design. It has been shown, however, that the application of hydrostatic pressure can raise T c — up to ∼164 K in the case of HgBa 2 Ca 2 Cu 3 O 8+δ ( ref. 3 ). Here we show, by analysing the uniaxial strain and pressure derivatives of T c , that compressive epitaxial strain in thin films of copper oxide superconductors could in principle generate much larger increases in the critical temperature than obtained by comparable hydrostatic pressures. We demonstrate the experimental feasibility of this approach for the compound La 1.9 Sr 0.1 CuO 4 , where we obtain a critical temperature of 49 K in strained single-crystal thin films — roughly double the bulk value of 25 K. Furthermore, the resistive behaviour at low temperatures (but above T c ) of the strained samples changes markedly, going from insulating to metallic.

The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. After placing a conical magnet (holmium) at the interface between the two, we detected a long-ranged supercurrent in the ferromagnetic layer. The long-range effect required particular thicknesses of the spiral magnetically ordered holmium, consistent with spin-triplet proximity theory. This enabled control of the electron pairing symmetry by tuning the degree of magnetic inhomogeneity through the thicknesses of the holmium injectors.